In many grinding procedures, in addition to the material to be ground, a gas current is also passed through the mill. The gas current serves either to convey or to cool the material to be ground. Regardless of the type of mill, after the grinding, the finely ground product must be separated from the gas current by means of a separator. Devices of various types are known for this purpose such as, for example, cyclones or sifters with rotating inserts. A simple separator which has proven its worth is based on the principles of allowing the gas current laden with the finely ground product in a housing to exit from a tubular insert and of returning the current along the outer side of the tubular insert. By sharply deflecting the gas current as it exits from the tubular insert, the finely ground product is separated from the gas current. Such a device is shown, for instance, in U.S. Pat. No. 4,579,288.
There are grinding procedures in which cooling is not only necessary during the grinding procedure, but rather, a low temperature also has to be maintained during the subsequent separation of the finely ground product and its storage. THis is the case, for example, with the grinding of biological substances whose cellular structure is disintegrated by the grinding. These biological substances, which are normally liquid, are first frozen by means of liquid nitrogen to form spherical pellets and subsequently ground at a temperature of, for instance, minus 70.degree. C. [minus 94.degree. F.]. This low temperature must be maintained since even a slight pressure stress already causes the product to thaw. This, in turn, leads to bridge formation and system closures. Grinding and storage at such a low temperature calls for a large volume of cooling gas. Due to economic considerations, this large volume of gas is recycled. This requires a thorough and almost complete separation of the finely ground product from the circulating gas current.
In this context, it has turned out that the separators having a sharp deflection of the gas current as it exits from a tubular insert--which are given preference due to their simplicity--are not completely satisfactory, since approximately 6 percent of the finely ground product still remains after the deflection of the gas current. Whereas such a level can be tolerated for many grinding procedures, it is not acceptable for costly products.